Metal-air batteries have long been a subject of research since they can be used in many applications, both as small batteries for watches, acoustical devices, etc., and as bigger batteries suitable for electric vehicles.
During the discharge of metal-air batteries, oxygen from the atmosphere is converted to hydroxyl ions in the air electrodes. The reaction in the air electrodes involves the reduction of oxygen, the consumption of electrons, and the production of hydroxyl ions. The hydroxyl ions migrate through the electrolyte towards the metal-negative electrode, where oxidation of the metal of the negative electrode occurs, forming oxides and liberating electrons. In a secondary (i.e., rechargeable) metal-air battery, charging converts hydroxyl ions to oxygen in the air electrode, releasing electrons. At the metal electrode, the metal oxides or ions are reduced to form the metal while electrons are consumed.
Metal-air batteries provide significant energy-capacity benefits. For example, metal-air batteries have several times the energy storage density of lithium-ion batteries, while using globally abundant and low-cost metals (e.g. zinc) as an energy storage medium. The technology is relatively safe (non-flammable) and environmentally friendly (non-toxic and recyclable materials may be used).